The present invention relates in general to testing equipment and methods in nuclear power plants, and in particular to a new and useful method and apparatus for detecting and measuring wear on the control rods for the fuel assemblies for nuclear power plants.
More and more frequently, control rod assemblies (CRA) are being thought of as components of pressurized water reactor (PWR) systems which require periodic non-destructive evaluation (NDE). In light of recent observations of breach-of-cladding, plant owners are under increasing regulatory pressure to determine the condition of their control components and to insure that shutdown margins are within specifications. Damage mechanisms include vibratory contact with support or guide components and absorber swelling due to neutron irradiation. As a result of either gross diameter increase in the cladding or leeching of the poison from the cladding, the ability to scram the reactor could be jeopardized.
During the operation of a PWR, the control rods are suspended above the fuel assembly with the individual pins contained within the brazement guide structure. The tips of the pins are captured in guide tubes located in the fuel assemblies. Coolant flow through the guide tubes causes the pins to vibrate, and the resulting mechanical contact between the control rod pins and the support structure/fuel assembly guide tubes induces localized wear on the outside surface of the individual pins. There are at least two different flaw types in the CRA. The first flaw type is a large volume wear mark caused by contact with the guide tube nut. The second type of defect is a small volume, axial groove caused by contact with the brazement support structure above the fuel assembly.
In the past, inspection of control rods, the primary component of rod cluster control assemblies (RCCAs) in pressurized water reactors, has been performed non-destructively, using various eddy current techniques. These eddy current techniques have been used to look for and measure cracks and wear marks that, if large enough, could render an RCCA unusable. The use of eddy current coils has been successful in determining the existence of a breach in cladding or the amount of material left on a control rod. In some situations, measuring the percent of material remaining has provided sufficient information for determining the useability of a control rod. In fact, pass/fail criteria has largely been based upon the kind of data (i.e. location, quantity and accuracy of data points) collected by eddy current inspection equipments.
Inherent in the making of eddy current measurements and especially in the interpretation of the measurements is the problem of material variability. Because eddy current measurements are based on the electrical properties of the material of which the control rods are made, a control rod calibration standard must be produced to replicate the actual control rod and damage mechanisms as closely as possible. Any difference in material or defect geometry has the potential to be a source of error in data collected on actual control rods. If a very precise method of measurement that is not dependent upon material properties could be integrated in a system to inspect control rods, the accuracy of measurement data could be improved.